The present invention relates to communication systems, and more particularly to a communication system employing a power control feature which allows the power of a signal transmitted from a first communication node to a plurality of secondary communication nodes to be maintained, in a closed loop fashion, within a predetermined range to thereby avoid interference with non-target nodes.
In a communication system where at least one node, hereinafter designated xe2x80x9cnode Axe2x80x9d, is in communication with at least one secondary node, hereinafter referred to as xe2x80x9cnode Bxe2x80x9d, the transmitting node A must transmit its signal with enough power to achieve an adequate signal-to-noise ratio at the receiving node B. However, if the signal transmitted from node A is transmitted with too much power, it may interfere with other nearby nodes or communication links that use the same frequency band. Moreover, a signal which is transmitted at a higher power level than necessary from node A to maintain closure of the communication link with node B would also make the device forming node A more vulnerable to hostile detection and interception in military applications. To achieve maximum use of scarce bandwidth and to minimize the probability of hostile detection and interception, the optimum solution would be for node A to transmit the signal with only enough power to maintain closure of the link to node B.
In a continuous radio frequency (RF) link using directional antennas, this optimal power level is achieved by using closed loop power control and bi-directional communication links. Node B monitors the incoming signal-to-noise ratio (or Eb/No). If the signal is too weak, then node B sends a message to node A telling it to increase power. If the signal is stronger than it needs to be, then node B sends a message telling A to reduce power. This allows node A to maintain the optimum power level despite changing conditions, such as rain absorption or multi-path fading. Concurrently, node A provides feedback to node B so that node B can control the power of its transmission.
In omnidirectional time division multiple access (TDMA) systems, this process has not been implemented. If power control is used at all in omnidirectional TDMA systems, it is of the open loop variety. One example is the power control scheme used in a TDMA cellular phone system. Each cell tower in this system transmits at constant power, with its power selected to cover a geographic area (i.e., a xe2x80x9ccellxe2x80x9d). Each phone in the system senses the incoming power from the cell tower. If the phone senses a strong signal it knows it is close to the tower so it uses low power for its transmission to the tower. If the phone senses a weak signal, it knows it is far from the tower, in which case it uses high power for its transmission. There is no closed-loop feedback from the tower to adjust the phone""s power level.
Another example is the LINK-16 military TDMA network. This system is meant to communicate across a theatre of war. Nodes typically do not know the location of other nodes with which they are communicating, so every node transmits with enough power to reach from one corner of the theater to another.
Some TDMA satellite uplinks with directional antennas use closed-loop power control to compensate for rain. This allows the uplink to maintain adequate signal-to-noise ratio at the satellite without exceeding the interference limits for nearby satellites. The uplink from each ground terminal goes to only one satellite, so the ground terminal only has to keep track of one power setting. Only the uplink uses power control. The downlink beam typically covers many receivers, some of which may be receiving signals through rain and some of which will not. Therefore, the downlink (a one-to-many transmission) uses constant power with enough power margin to penetrate the rain an acceptable fraction of the time.
In a military application, wherein a large theater must be covered, node A may form TDMA links with over a dozen or more other secondary nodes (i.e., xe2x80x9cBxe2x80x9d nodes). Links from a single node A may be as short as one kilometer to a given node B or as long as 900 km to a different node C, with both distances occurring simultaneously. When the 900 km link must penetrate heavy rain and the one km link passes through clear sky, the power needed to close the long link may be nearly one million times greater than the power needed to close the short link. The communication links are formed preferably using phased array antennas manufactured by the assignee of the present application, which produce very narrow beams. If the short link to node B transmits at the same power as the long link, then nodes anywhere in close proximity to node B may be overwhelmed by the power of the A to B link, and thereby suffer interference from the A to B link.
The foregoing limitations thus give rise to a need for a communications scheme which assigns only as much power to a transmit beam from the first node as necessary to maintain closure of a communication link with a given second node. More specifically, there is a need for a power control scheme for use with a communications network in which a transmitting node (i.e., node A) is able to receive feedback regarding a signal strength of a receive signal received by a secondary node (i.e., node B), and wherein a power level of subsequent transmissions from node A are modified such that only a level of power is used with the transmitted signal which is needed to maintain closure of the communication link with the desired secondary node. Such a system would provide a closed loop power control system which would allow the power of signals transmitted from one node to be continually adjusted, as needed, to maintain closure of a communication link with a second node without potentially causing interference with other nodes in proximity to the second node.
The above and other objects are provided by a communication system incorporating a power control system in accordance with a preferred embodiment of the present invention. The communication system is formed in accordance with a multiplexing scheme in which at least one or primary node is able to communicate with a plurality of secondary nodes. In one preferred form, a TDMA communication system is implemented in which each secondary node is assigned a designated time slot in a TDMA cycle. The primary node maintains a data file associated with each secondary node. Each data file includes a record of the power level of each transmission (or xe2x80x9cburstxe2x80x9d) made to each secondary node, together with a time interval during which the transmission is made. Each secondary node, upon receiving the transmission from the primary node, responds with a transmission which includes a power correction message. The power correction message indicates the needed change in the power level to just maintain closure of the communication link. The power correction message is received by the primary node and the power level of a subsequent signal to be transmitted from the primary node to that particular secondary node is either reduced or increased in accordance with the requested change in power.
Each data file maintained by the primary node, as mentioned above, includes a time interval for each transmission. In a preferred method, a secondary node transmitting back to the primary node also sends an indication of the time that the transmission from the primary node was received, as well as a xe2x80x9ctime stampxe2x80x9d indicating the time that the secondary node transmitted back to the primary node. Using this information, the primary node is able to xe2x80x9cmatchxe2x80x9d the signal strength information received from a given secondary node with a particular transmission which it has previously sent, and can therefore accurately determine whether or not the power level of a subsequent transmission to the secondary node needs to be adjusted. The time stamp transmitted by the secondary node also allows the primary node to update the time interval it maintains as to when a given transmission having a given power level has occurred.
The present invention thus enables a large plurality of communication links to be maintained with a plurality of secondary nodes, and wherein each secondary node provides signal strength information to a primary node with which it is in communication with to allow a closed loop power control scheme to be implemented. The closed loop power control scheme provides accurate, real time control over the power level of transmissions from a primary node to each one of a large plurality of secondary nodes. Thus, only that level of power needed to maintain closure of the communication links with each of the secondary nodes is used by the primary node.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.